This invention relates to a device, which is assembled in a stereophonic broadcast receiver for detecting an identifying signal indicating that the received broadcast wave is a stereophonic broadcast wave (hereinafter referred to as stereo identifying signal) from the received broadcast signal.
As the well-known stereophonic broadcast system, there is an AM stereophonic broadcast system based on the Magnavox system. The transmission signal in the Magnavox system is expressed as EQU E(t)=[1+m{L(t)+R(t)}] cos [.omega..sub.c t+B{L(t)-R(t)+A cos .omega..sub.0 t}] (1)
where L(t) and R(t) are left and right channel signals, n is the degree of AM modulation, B is the index of PM modulation which is equal to .+-.1 rad. max, A is the index of FM modulation (amplitude component of the identifying signal), .omega..sub.0 is equal to 2.pi.f (f=5 Hz) and .omega..sub.c is equal to 2.pi. (carrier frequency).
In the equation (1), A cos .omega..sub.0 t represents the stereo identifying signal component. It will be seen that this component is produced as a result of FM modulation of the carrier wave. The stereo identifying signal is modulated by a frequency of 5 Hz, and the frequency deviation of the carrier wave by the modulation is set to 0 to .+-.20 Hz. The FM modulation may also be thought as a phase modulation, and the deviation of the phase modulation is .+-.20.div.5=.+-.4 (rad.). The receiver is set either in a stereophonic or monophonic receiving mode depending upon whether such a stereo identifying signal is contained in the received broadcast signal. When receiving the broadcast wave, the receiver is desired to provide quick response and reliable action of switching between the stereophonic and monophonic receiving modes. However, in the broadcast system mentioned it is considerably difficult to let the receiver reliably and quickly respond to the stereophonic broadcast. This is because of the facts that the frequency of the stereo identifying signal is very low, namely 5 Hz, and also very large DC level variations are introduced into the demodulated output at the time of the tuning operation due to an S-shaped characteristic of the detector.